The present invention relates to ignition apparatus for rotary piston engines, and more particularly to ignition apparatus for rotary piston engines which use glow plugs as ignition means for igniting the air-fuel mixture.
In a rotary piston engine, in general because of the complex contour of a combustion chamber, the combustion rate of the air-fuel mixture is low. Accordingly, it is necessary to ignite the air-fuel mixture at a very early time point before top dead center in order to save the expense of fuel. When a spark plug is used, however, gas cleaning in an ignition hole in which the spark plug is mounted is insufficient as the rotor is further from top dead center. For example, near 40.degree. before top dead center (crank angle), it is difficult to ignite the air-fuel mixture by the influence of remaining gas. It is, therefore, practically difficult to establish very early ignition timing to the spark plug.
On the other hand, the inventors have experimentally proven that when an electrically heated plug is used instead of the spark plug, ignition occurs in effect between 50.degree. to 30.degree. before top dead center enabling very stable operation which greatly reduces fuel expense while maintaining a good emissions level.
However, the electrically heated plug inherently requires a certain preheating time after energy is applied to the plug for the igniting portion to reach a temperature allowing ignition of the air-fuel mixture. If cranking of the engine is effected during the preheating period, the ignition does not occur and the power is wasted resulting in shorter battery life. Accordingly, in the start-up of the engine, the start must be delayed until after a preheating period so that the plug is ready for ignition.
Further, in igniting the air-fuel mixture by the glow plug, it is necessary to maintain the glow plug at a temperature higher than an ignitable temperature (approximately 1000.degree. C.) and at the same time it is desirable to prevent the glow plug from being heated above a necessary temperature in order to improve the durability of the glow plug. It is usually desirable to maintain the glow plug at a temperature below approximately 1100.degree. C.